1. Field of the Invention
The present invention relates to an off-power line switching mode power supply which converts primary rectified, unregulated high voltage to a number of regulated d.c. secondary voltages and a low frequency, high voltage output suitable for enabling ringing of standard telephones.
2. Description of the Prior Art
Typical prior art off-power line power supplies had large choke fed secondary outputs "slaved" from the highly regulated output of one of the d.c. output voltages of the plurality of d.c. output voltages provided by the power supply. Such a design, however, would not permit large load variations of the highly regulated output without causing excessively high primary peak current, of a potentially destructive nature, from damaging the switching transistors of the power supply.
Certain prior art off-power line power supplies have attempted to correct this problem by the use of a push-pull arrangement of bipolar power transistors with a center-tapped transformer primary winding. However, if an imbalance in the conduction times of the transistors occurs, a d.c. current can pass through the transformer primary causing impending core saturation and device destruction. Such an imbalance results from the variation of the storage time of bipolar transistors from device to device.
Other prior art off-power line power supplies have combined the use of a half-bridge inverter with saturable reactors. However, transformer core saturation and transistor damage result if symmetry of operation of the saturable reactors is not achieved. Symmetry control is attempted at the cost of d.c. losses and circuit complexity.